In this document, a mineral molding is a molding produced from a mixture (a mortar) of a mineral binder, water, aggregates and, if desired, auxiliaries by, after shaping, the mineral binder/water mixture curing as a function of time, if desired under the action of elevated temperature. Mineral binders are generally known. They are finely divided inorganic materials such as lime, gypsum, clay and/or cement which are converted by mixing with water into their ready-to-use form, with the latter solidifying like stone as a function of time when left, in air or even under water, if desired under the action of elevated temperature.
The aggregates generally comprise granular or fibrous natural or synthetic rock (gravel, sand, mineral fibers), in special cases even of metal or organic aggregates or mixtures of the specified aggregates, having grain sizes or fiber lengths which are matched to the respective application in a manner known per se. Frequently, colored pigments are concomitantly used as aggregates for the purpose of imparting color.
Suitable auxiliaries are, in particular, those substances which accelerate or retard curing or influence the elasticity or porosity of the solidified mineral molding. These are, in particular, polymers as are known, for example, from U.S. Pat. No. 4,340,510, GB-B 15 05 558, U.S. Pat. No. 3,196,122, U.S. Pat. No. 3,043,790, U.S. Pat. No. 3,239,479, DE-A 43 17 035, DE-A 43 17 036, JP-A 91/131 533 and other documents.
Examples of the mineral moldings to be preserved according to the invention are concrete pipes as are used for the transport of sewage (cf., for example JP-A 55/44 273), concrete roofing tiles (cf., for example DE-A 21 64 256 or DE-A 39 01 073) or curb-stones, steps of stairways, floor slabs or baseplates on the basis of mineral binders.
A disadvantage of mineral moldings is that under the action of weathering (in particular the action of water) the cationic constituents such as Ca.sup.2+ are leached out over the course of time, which reduces their strength. A further drawback of mineral moldings is the occurrence of efflorescence. This is presumably attributable to the fact that the mineral binders contain polyvalent cations such as Ca.sup.2+ in an alkaline environment. Reaction with the carbon dioxide of the air can thus form sparingly water-soluble, ugly white lime spots on the surface of the mineral molding. The appearance of efflorescence can occur both during the curing of freshly prepared mineral moldings and also after the action of weathering on previously cured mineral moldings.
Attempts have long been made to counter the abovementioned draw-backs by sealing the mineral molding on its surface by application of a suitable coating.
Attempts were first made to preserve mineral moldings by coating with surface coatings based on vinyl chloride copolymers in organic solvents.
A disadvantage of this procedure is that it can be used only for dry mineral moldings since, on mineral moldings which are still moist, the adhesion of the surface coating is poor and, in addition, porous films are formed.
The abovementioned disadvantages of the organic surface coatings were relatively quickly overcome by sealing the surface of the mineral moldings by coating with aqueous compositions containing an aqueous polymer dispersion as film-former.
Dispersion polymers recommended for this purpose were styreneacrylic ester copolymers, styrene-butadiene copolymers, homopolymers and, in particular, copolymers or terpolymers of vinyl acetate with olefins, vinyl halides and/or (meth)acrylic esters, homopolymers or copolymers of vinyl propionate, (meth)acrylic ester homopolymers or copolymers, etc. (cf., for example DE-A 21 64 256, page 4).
However, the resulting coatings were not satisfactorily able to prevent the cationic constituents passing through. The Swiss Patent 446 720 recommends as a remedy the addition to the aqueous polymer dispersions of water-soluble salts or acids which form sparingly soluble salts with the cations of the mineral binders.
However, the action of these additives is limited since they are, for example, washed by rain from the films of the aqueous polymer dispersions as a function of time and thus only short-term protection is provided.
DE-A 39 01 073 and DE-A 38 27 975 recommend the preservation of mineral moldings by coating the corresponding surfaces with an aqueous composition containing as binder an aqueous polymer dispersion whose dispersed polymer contains from 0.5 to 5% by weight of at least one of the monomers acrylic acid, methacrylic acid, vinylsulfonic acid, itaconic acid, crotonic acid, maleic acid and fumaric acid in free-radically copolymerized form.
A disadvantage of these coatings of mineral moldings is that their preservative action is not completely satisfactory.